A communication system comprises signalling points or nodes, such as user terminals, different exchanges, routers, switches, links, stations and so on and an appropriate communication media between the signalling points. Signalling points may also be situated within an element of the communication system, wherein the communication may occur with the element. The communication media may comprise, for example, a wired interface, a radio interface or an optical interface. The communication may be carried by analogue or digital signals or a combination of these, such as digitally-modulated analogue signals.
Amplification is required in various communication applications. For example radio frequency signals transmitted between signalling points in a radio communication system may need to be amplified during some stage of the transmission and/or reception. The signalling points may be, for example, a transmitting station and a receiving station or an intermediate node of the communication system. The amplification of the signals is required since the amplitude of a signal tends to be attenuated during the transmission of the signal between the signalling points, thereby decreasing the quality of the transmission. Also, noise is added to the signal from other sources as well as from the transmitting and receiving and the possible intermediate apparatus itself. The communication system is thus provided with amplifying means to compensate for the attenuation and increase the signal-to-noise ratio of the signal by amplification of the signal.
Amplifiers intended to cover a range of frequencies should provide linear performance across the designated frequency band. Any amplifier introduces linear, or AM-PM (amplitude modulation-phase modulation) distortion, where amplitude variations in the input signal cause undesirable phase variations in the output signal, and intermodulation, or AM-AM distortion, causing mixing between the different frequency components present. An example of so-called third-order and fifth-order distortion components appearing around a simple two-tone signal at frequencies f1 and f2 is shown by FIG. 1. Although other distortion components are generated, these tend to be produced at frequencies that are significantly higher than the desired signals, allowing their easy removal with filtering. As illustrated, the magnitudes of the third-order distortion components are normally greater than the fifth-order components. Even though not illustrated in FIG. 1, seventh-order (or even higher odd-order) distortion components may also appear around the carrier signals. However, the third-, and to a lesser extent, the fifth-order distortion will dominate the nonlinearity of an amplifier in most cases.
The non-linearity of an amplifier is caused by the finite output power limitation and non-ideal transfer function of the amplifier. Therefore, it is often desirable to provide amplifiers with some kind of linearizing circuitry to reduce the distortion that is introduced. A straightforward solution to the linearity problem exploits the fact that the non-linearity increases with the output power level of the amplifier. Thus, if the input level is reduced, or “backed-off”, the amplifier is arranged to operate only within its more linear region. However, this approach is not considered to be desirable in most applications as it fails to utilise the full range of available output voltage-swing and is therefore not power-efficient.
There are a number of well-established linearization techniques, which have been proposed over the years. One of the most well-known prior art techniques is referred to as feedforward (F/F) linearization. The configuration and basic operation of typical F/F circuitry is shown in FIG. 1. In the shown F/F arrangement an input signal consisting of two closely spaced tones is first sampled by a directional coupler in a location before the amplifier. The separated fraction of the clean, undistorted input signal is passed through an amplitude and phase-shifting network while the input signal is passed through the main amplifier. After the amplification, a further sample is separated from the amplified signal, said further sample including also the distorted components of the amplified signal. The two sampled signals are combined in a hybrid combiner in exact antiphase and with equal amplitudes in order to cancel the original two-tone signal, leaving an “error” signal that consists of only those distortion components that were introduced by the amplifier. The error signal is then amplified by an error amplifier and subsequently combined with the distorted output of the main amplifier, again in precise antiphase and with equal magnitude, removing the extraneous distortion components and leaving a clean amplified signal.
Feedforward linearizers are difficult to realize because all the components operate at radio frequency (RF), and the phase and amplitude tolerances of the two cancellation loops are very tight and susceptible to changes in temperature and ageing. In order to combat the aforementioned problems, a relatively complex control mechanism has to be added in order to maintain acceptable performance. The feedforward linearizers are also temperamental, complex and inefficient to operate. Due to the above reasons, feedforward linearizers are relatively expensive and may thus be unsuitable for some applications requiring linearization.
Another prior art linearization technique is predistortion. In predistortion the input signal is deliberately distorted prior to being inputted to the amplifier in a manner that is contrary to that distortion that the signal will experience in the amplifier itself, resulting in a “cleaner” signal. Both analogue and digital predistortion have been suggested. The predistortion systems may be constructed so as to form an open loop predistorter or a closed loop (i.e. adaptive) predistorter. The latter has the advantage of being able to adjust for device variations with temperature and time.
Predistortion linearizers operate to predistort the high-frequency carrier itself, and therefore suffer many of the drawbacks of F/F linearizers. The analogue predistortion linearizers operate only over a relatively small power range. The current digital predistortion linearizers employ a complex structure, being difficult and expensive to realize. Despite the complexity, even the digital predistortion linearizers have shown only a limited distortion improvement.